This application relates to optical pick-up video disc players and more particularly to radial tracking servo systems using elasto-optic devices for causing periodic lateral displacement of the light beam in an optical pick-up video disc player as it reads the record track.
In an optical pick-up video disc record player, a focused light beam follows a spiral track as the disc rotates. The track usually consists of depressions, which may but need not be in the form of uniform-depth pits of varying lengths, having a radial width the order of one micron. Adjacent turns of the track are spaced from each other by a distance of from one to four microns. The normal eccentricity of the disc, which is typically 50 microns but possibly as much as 100 microns, makes it necessary to provide a radial tracking servo system. Proper operation of such a servo system requires a sensing signal, that is, a control voltage proportional to any excursion of the focused spot from the center of the record track.
There are several methods known for developing the required sensing signal. One method that has been proposed is called spot wobble. The focused light beam spot is moved periodically back and forth across the track in a radial direction at a rapid rate. Its excursion in each direction during this periodic motion is only a fraction, such as 20 percent, of the light spot size which typically may be one micron in diameter. The intelligence signal derived from the record track, which contains both DC and radio frequency components, is a maximum when the spot is centered upon the track; in that case, the rapid periodic motion of the spot is also centered about the track, with the result that the fluctuation of signal amplitude produced by the spot wobble is small and has no component at the wobble frequency. If, however, the spot drifts off the center of the track, then the periodic excursions to one side will produce a significant drop in amplitude while the excursions to the other side will produce no such drop and may even produce an increase in the signal if the spot is sufficiently far off-center. Thus the magnitude and polarity of any signal amplitude fluctuation at the wobble frequency is indicative of the departure of the spot from its correct position on the record track. To generate a sensing signal, such signal amplitude fluctuation must be derived from the photoreceptor output and its polarity compared with a reference signal at the wobble frequency. This comparison is done in a synchronous detector; the reference signal is taken from the signal source that produces the spot wobble in the first place. The output from the synchronous detector may then be used as the sensing signal for the radial tracking servo system.
As stated previously, the primary function of the radial tracking servo loop is to compensate for the radial motion of the record track caused by record disc eccentricity. This motion occurs typically at a rate of 30 Hertz and may have an amplitude as large as 100 microns. Assuming that it is desired that the focused spot stay within 0.1 micron of the track center, the servo loop must have a gain of 1000 at a frequency of 30 Hertz. At higher frequencies, the loop gain may be permitted to decrease; it must finally drop to unity at some frequency f.sub.O. There is an additional requirement imposed upon the radial tracking servo system if it is desired to provide for stop-frame or slow-motion operation; in such operation, the spot is made to switch rapidly from one turn of the track to another, and it is desired that operation in the new turn become stable within no more than about 100 microseconds. This requires that the radial tracking servo loop gain be above unity up to frequencies of several thousand Hertz.
If a spot wobble system operating on a wobble frequency f.sub.w is used and the frequency f.sub.w is too close to f.sub.O, it becomes difficult to suppress the residual wobble-frequency component from the output of the synchronous detector without incurring undesirable phase delay at the frequency f.sub.O. For this reason, the wobble frequency f.sub.w should be chosen several times higher than f.sub.O. Practical experience has shown that it is desirable to avoid the horizontal scanning frequency of 15,734 Hertz and its harmonics. Frequency bands such as 20 to 27, 36 to 43, or 52 to 58 kilohertz are particularly suitable for f.sub.w.
Deflection of the light spot by 0.2 microns represents only about one five-hundredth of the deflection capability of a typical radial tracking system. It has therefore been proposed to produce the spot wobble with the same mechanism -- usually a movable mirror -- that is used for radial tracking. This however is impractical at the high wobble frequencies mentioned, because of the excessive acceleration that would be needed. Acceleration equals the product of excursion and the square of the angular frequency. Hence a deflection of 0.2 micron at 20 kilohertz represents 900 times the acceleration required to produce a deflection of 100 microns at 30 Hertz. This is an impossible requirement for the usual radial tracking mechanism.
It has been proposed to produce the wobble frequency deflection by means of a separate mirror mounted on a support that is constructed so as to be resonant at the wobble frequency in a flexural mode of vibration, which results in tilting of the mirror. A device of this kind has the disadvantage that the mirror necessarily redirects the light beam; therefore the orientation of the mirror is critical and must be adjusted precisely.